This invention relates to the art of data transmission and more particularly to a method and apparatus for storing surplus ADSS cable along an aerial route. ADSS is an acronym for "all-dielectric self-supporting". Such cable has a strong non-metallic sheath which supports the optical fibers making up the cable. ADSS cable may also have a reinforcing strand at its core. All-dielectric cable has the advantage that it can be used in close proximity to electrical power lines, whereas conventional communications cable are required to be run in a separate zone, usually at least forty inches below the power cables.
Optical cable is vulnerable to damage (fiber breakage) from bending and twisting, and performs best when sag and twisting are minimized. Cable manufacturers specify a minimum bend radius which must be strictly observed to avoid fiber damage. Currently, as a rule of thumb, the bend radius cannot be less than ten times the cable diameter for an unstressed cable, fifteen times the diameter for a stressed cable. This rule of thumb may change as cable construction changes; the trend appears to be toward cables having smaller minimum bend radii. In any event, devices used to store surplus cable, must protect the cable from bending too sharply, that is, at less than the specified minimum bend radius.
Fiber optic cable is typically installed on aerial routes in very long lengths so as to minimize the number of splices, each of which degrades optical signals. To allow for rerouting, such as pole movement, and for repairs, it is important to provide slack in the form of surplus lengths of cable at intervals along the route, so that entire long lengths of cable do not have to be taken down when minor repair or rerouting is required. The more frequent the storage interval, the less the probable length of cable which must be rehung if rerouting is necessary. The surplus cable may be stored either below ground, or along the aerial route.
In the underground storage method, the cable is routed from its aerial attachment location down a pole and into an underground enclosure. The cable is coiled within the enclosure, and then routed back up the pole to continue along the main cable route. This method is presently the predominantly accepted method for storing surplus ADSS cable, but it is has several disadvantages.
First, underground storage requires an expensive, watertight, underground enclosure and necessitates the expense of excavation. Moreover, an unobstructed excavation site is not always available at the exact pole location where the cable must be stored.
Second, an enclosure sometimes does not provide sufficient space to store the required length of cable in a manner that renders the cable easy to access and manage when access is necessary. If the pole has to be relocated subsequently, there is an additional moving expense of excavation at the new pole site to install the container.
Third, the fact that the cable must be routed up and down the pole from the aerial attachment location to the underground enclosure makes the cable vulnerable to damage from collisions (e.g., auto accidents, being struck by mowers, etc.) and from vandalism. Any such damage to ADSS cable would require potentially extensive and costly cable replacement attended by unacceptable loss of service. Clearly, an aerial method of storing ADSS cable would avoid the drawbacks of the underground storage method just described.
For aerial storage, it has been proposed to run surplus optical cable around snowshoe or teardrop shaped devices to avoid overbending. Prior U.S. Pat. No. 5,092,663 (Hivner) and 5,408,517 (Kaplan) exemplify prior devices. These devices, however, were designed for pre-ADSS fiber optic cable, which does not have sufficient strength to support itself, and must be lashed to or otherwise supported from a steel or metallic messenger cable. While prior inventions are acceptable for storing messenger-supported fiber optic cable, no provision has been offered which would make such inventions suitable for use with ADSS cable, which must be protected at all times not only from overbending, but also from incidental contact and abrasion.
In the installation of messenger-supported fiber optic cable, attachments of any necessary hardware such as that described in the patents mentioned above may be made directly to the steel messenger cable without any danger of damaging the fiber optic cable. In the case of ADSS cable, however, the supporting member is the outer sheath of the cable, which is subject to possible damage if any attachment which is not properly designed is made directly to the cable.
A problem related to that of surplus cable storage is that of supporting splices between cables. Splicing is usually done at ground level in a truck or trailer containing special splicing apparatus. Therefore, enough excess cable must be provided at the splice so that it can reach the ground. Additionally, once the splice has been made, it is secured in a box known as a "splice closure", which may weigh as much as 35 pounds. Because of their weight, such closures have been mounted on the pole in the past, but this creates a confusion of cables near the pole, where they might be damaged, and without any guarantee of minimum bend radius protection. It would be better to support the splice box from the cable, if one could avoid injuring the cable.